1. Field of the Invention
The present invention relates to control systems for controlling motor speed.
2. Description of the Related Art
A known motor speed control system is shown in FIG. 7 and generally includes an encoder 102 for detecting the rotational speed of a motor 101, a frequency/voltage converter 103 for converting an output signal from the encoder 102 into a voltage signal V that represents the rotational speed, and a setting unit 104 that can output voltage signal Vs corresponding to a set value of the rotational speed.
The encoder 102 is configured to output a pulse generation signal at each time that the motor 101 has rotated by a predetermined angle. The frequency/voltage converter 103 generates a pulse having a pulse width at each time that the pulse generation signal is produced at the encoder 102. Therefore, the frequency/voltage converter 103 produces a series of pulses. The converter 103 includes a smoothing circuit that converts a series of pulses into a voltage value corresponding to the pulse density. Thus, as the rotational speed of the motor 101 increases or decreases, the pulse density may be increased or decreased. Therefore, the voltage signal V after being smoothed by the smoothing circuit varies in proportion to the rotational speed of the motor 101.
Then, a comparator 105 compares the voltage signal V (i.e., the output signal from the frequency/voltage converter 103) corresponding to the rotational speed of the motor 101 with the voltage signal Vs corresponding to the set rotational speed of the motor 101. Subsequently, a controller 106 controls the supply of power to the motor 101 such that the difference (Vs−V) between the voltage signal V and the voltage signal Vs is decreased.
However, because the frequency/voltage converter 103 of the known system 100 converts a series of pulses into the voltage corresponding to the pulse density by means of the smoothing circuit, delay in transmission of signal may occur due to the smoothing circuit. Therefore, it is necessary to provide additional circuit for compensating for the delay in phase caused by the smoothing circuit, resulting in increase of the cost of the control system 100. In addition, even if the compensation has been made, it is likely that the usable speed range is limited. Further, even if a small-bit low cost microcomputer (such as 8-bit to 16-bit microcomputers) is included for digitalizing the analogue value, it is difficult to obtain a voltage signal V in a desired accuracy from the smoothing circuit.
In order to solve this problem, Japanese laid-Open Patent Publication No. 63-213485 has proposed to control the rotational speed of the motor without using the smoothing circuit. A speed control system disclosed in this publication is shown in FIG. 6(A) and is labeled with reference numeral 120. According to this speed control system 120, a time T between pulse signals (i.e., a pulse interval T) from the encoder 102 is determined by a timer counter 123. Then, a count value C corresponding to the determined pulse interval T is compared with a count value C3 corresponding to a pulse interval Ts (i.e., a set value Ts), and subsequently, a controller 106 controls the power supplied to the motor 101 such that a difference (Cs−C) decreases.
Here, the pulse interval T decreases as the rotational speed of the motor 101 increases, and the pulse interval T increases as the rotational speed of the motor 101 decreases. Thus, the pulse interval T is inversely proportional to the rotational speed of the motor 101. Because the speed control of the motor 101 is performed using the count value C corresponding to the pulse interval T as it is, without using the inverse number of the pulse interval, it is possible to use a small-bit microcomputer.
However, according to the speed control system 120, a timer counter 123 counts the pulse interval T of the pulse signals from the encoder 102 based on a fixed oscillating frequency t. For example, if the motor 101 rotates at a speed of 100 rps, and the encoder 102 generates one pulse per revolution, a pulse interval T1 in this case is calculated to be 1/100 (seconds) (T1= 1/100). Therefore, if the timer counter 123 counts using a time interval t (t=16 μs/count), the count value C of the timer counter 123 is calculated to be 625 (C=T1/t). On the other hand, if the motor 101 rotates at a high speed, for example at a speed of 650 rps, a pulse interval T2 in this case is calculated to be 1/650 (seconds) (T2= 1/650), and the count value C of the timer counter 123 is calculated to be 96 counts (C=T2/t). Then, the count value C of the timer counter 123 is compared with the count value Cs corresponding to the set value Ts (Cs=Ts/t).
In the case that the set rotational speed Ns of the motor 101 has been decreased by 4% when the rotational speed N of the motor 101 is high (N=650 rps, C=96 counts), the set rotational speed Ns is 624 rps (Ns=N−0.04N=650−0.04*650=624). If the set rotational speed Ns is 624 rps, the set value Ts is 1/624 (seconds), and the counted value Cs of the counter C (C=Ts/t) is 100. Thus, the difference between (Cs−C) is calculated as “(Cs−C)=(Ts/T)−(Ts/t)=100−96=4.”
On the other hand, in the case that the set rotational speed Ns of the motor 101 has been decreased by 4% when the rotational speed N of the motor 101 is low (N=100 rps, C=625 counts), the set rotational speed Ns is 96 rps (Ns=N−0.04N=100−0.04*100=96). If the set rotational speed Ns is 96 rps, the set value Ts is 1/96 (seconds), and the counted value Cs of the counter C (C=Ts/t) is 651. Thus, the difference between (Cs−C) is calculated as “(Cs−C)=(Ts/T)−(Ts/t)=651−625=26.”
Therefore, although the reduction ratio or the deviation ratio is 4% both in the cases of the high rotation and the low rotation of the motor 120, the difference in counts (Cs−C) is largely different between the cases of the high rotation and the low rotation. Thus, even if the deviation ratio is the same, the difference in counts (Cs−C) is large in the case of the low speed operation while the difference is small in the case of the high speed operation. Therefore, it is not possible to properly accurately perform the control over the low speed operation and the high speed operation.
Thus, there is a need in the art for a motor speed control systems that can properly perform a motor speed control with the same accuracy over the low speed rotation and the high speed rotation.